Capacitor charge circuit

ABSTRACT

A signal comparator compares a reference voltage with a related voltage. The related voltage is related to the higher of the actual voltage on the capacitor or a charging signal voltage. The signal comparator output signal is controlled so that it can fire the capacitor charging SCR only on the downslope of a rectified wave so that the voltage to the capacitor can be no higher than the voltage at which the SCR was triggered.

United States Patent inventor Malcolm M. Oakes Torrance, Calif. Appl.No, 819.831 Filed Apr. 28, 1969 Patented Aug. 10,1971 Assignee HughesAircraft Company Culver City, Calif.

CAPACITOR CHARGE CIRCUIT 7 Claims, 1 Drawing Fig. 0.8. CI. 307/252 .1,219/136, 307/246, 328/67, 328/72 Int. Cl H03k 17/00, H03k 1/00 Field ofSearch 307/25250, 252.51, 246; 328/67, 72; 3l5/23824l; 219/136 [56]References Cited UNITED STATES PATENTS 3,049,642 8/1962 Quinn 307/2523,139,585 6/1964 Ross 328/67 3,258,697 6/1966 Guettel .4 328/723,354,288 11/1967 ONeal 328/72 Primary Examiner-Donald D. ForrerAssistant Examiner David M. Carter Attorneys-James K. Haskell and AllenA. Dicke, Jr

ABSTRACT: A signal comparator compares a reference voltage with arelated voltage. The related voltage is related to the higher of theactual voltage on the capacitor or a charging signal voltage. The signalcomparator output signal is controlled so that it can fire the capacitorcharging SCR only on the downslope of a rectified wave so that thevoltage to the capacitor can be no higher than the voltage at which theSCR was triggered.

Trigger Malcolm M. Ookes INVENTOR.

ALLEN A. DICKE, Jr.,

AGENT.

CAPACITOR CHARGE CIRCUIT BACKGROUND This invention is directed to acapacitor charge circuit and particularly to a capacitor charge circuitwhich rapidly brings a capacitor to be charged up to the voltagecorresponding to the desired charge. It is particularly useful inquickly charging the capacitor in a capacitor discharge welding system.

Capacitor welding systems are well known. They employ the charge on acapacitor to supply the energy necessary for welding, and are oftenemployed in other types of heating. Usually a stepdown transformer isemployed between the capacitor and the welding electrodes so thatcapacitors of reasonable capacity may be employed, and high currents atfairly low voltages are obtained at the welding electrodes. The amountof welding energy is related to the voltage to which the capacitor ischarged and to the size of the capacitor. Thus, for a particularcapacitor, it is necessary to charge the capacitor to the correctvoltage level to obtain the correct amount of welding energy. Since theamount of energy stored in the capacitor is related to the voltagethereon, previous systems have employed capacitor charging circuitswhich employ the capacitor voltage as the only feedback signal. However,with these systems the proper capacitor voltage was achievedasymptotically, and accordingly a considerable amount of time wasrequired to obtain the full desired voltage. Accordingly, on someautomatic welding jobs, the weld cycle time was limited by the capacitorcharging rate instead of by other limiting conditions.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to a capacitorcharging control circuit. The charging control circuit employs switchingmeans to control rectified current flow vto the capacitor to be charged.First and second signals are compared. The first signal is a rectifiedsine wave signal at lower voltage than the switching means input. Thesecond signal is the voltage on the capacitor. The higher of these twofirst and second signals is employed in a signal comparator, whichcompares that signal to a reference signal. The signal comparatorcontrols the switching means.

Accordingly, it is an object of this invention to provide a signalcomparator circuit to control the charging of a capacitor. It is afurther object of this invention to provide appropriate signals to asignal comparator input so that a capacitor charging switching means iscontrolled to quickly charge the capacitor to the desired value. It is afurther object to provide control for a capacitor charging SCR so thatit only fires on the downslope portion of a rectified sine wave. It isstill another object of this invention to provide variable control ofcapacitor charging by controlling the signal inputs to a signalcomparator. It is a further object to provide a capacitor charge circuitwhich employs first and second signal comparators so that the propersignals for capacitor charging are compared to control a capacitorcharging SCR.

Other objects and advantages of this invention will become apparent froma study of the following portion of the specification, the claims andthe attached drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawing is aschematic electric circuit diagram of the capacitor charge circuit ofthis invention as employed with a capacitor welding circuit.

DESCRIPTION Referring to the drawing, the capacitor charge circuit ofthis invention includes power circuit and control circuit 12. Powercircuit 10 has a power input including transformer secondary 14. Thesecondary 14 is energized from any convenient power supply, and for thepurposes of illustration, it is assumed that the power supply operatesat 60 cycles per second. However, it is clear that this frequency ismerely illustrative, and any convenient frequency can be employed.Transformer secondary 14 has its extremities connected to diodes 16 and18, which have their outputs connected together to unregulated positivebus 20. Transformer secondary 14 is center tapped with its centerconnected to negative bus 22. Bridge or single phase rectifiers couldalternatively be employed.

Unregulated positive bus is connected to and terminates at SCR 24 whichcontrols the charging of capacitor 28. The output of SCR 24 is connectedto regulated positive bus 26.

Capacitor 28 is the capacitor upon which the voltage level is'controlled to control the amount of charge thereon. Capacitor 28 isconnected between buses 22 and 26. Paralleled across capacitor 28 istransfonner primary 30 which is serially connected through SCR 32. Thefiring of SCR 32 controls the discharge of capacitor 28 throughtransformer primary 30.

Transformer secondary 34 is energized by magnetic coupling from primary30 and has its ends connected to welding electrodes 36 and 38.Preferably, at least one of these electrodes is movable so that they canbe engaged against work pieces 40 for the purpose of welding theseworkpieces together.

With respect to control circuit 12, first signal line 42 is connected tobe energized from unregulated positive bus 20 through resistor 44. Bus20 carries a waveform of full wave rectified sine wave, and resistor 44such that the peak value of voltage in line 42 is about one-third thatof the peak value of voltage in line 20. The two lines are insynchronism, both carrying rectified sine waves, and differ only inamplitude. It is desirable to fire SCR 24 only on the downslope of therectified sine waves so that the voltage does not rise after the SCR isfired. This prevents over voltage from overcharging capacitor 28, whichcould occur if the SCR were fired upon the rising sine wave.

Such lockout is provided by 0 to pi/2 lockout circuit 46. Capacitor 48is connected to line 20 and to the base of transistor 50. The emitter oftransistor 50 is connected through line 52 to ground and through diode54. Diode 54, in turn, is connected to the base of transistor 50. Thecollector of transistor 50 is connected to the input of SCR triggercircuit 56, which in turn controls firing of SCR 24. Trigger circuit 56is conventional, such as can be selected from pages 4l-69 of GeneralElectric Co. SCR Manual, Third Edition, 1964.

Pi/2 lockout circuit 46 is such that upon rising voltage in line 20,transistor 50 is forward biased to connect SCR firing signal line 58 toground through line 52. Thus, no input to line 58 can be effective uponrising voltage in line 20. However, when voltage starts falling in line20, transistor 50 is biased off to remove the ground connection from SCRfiring signal line 58. Capacitor 48 recharges through diode 54 on thedownslope. The Pi/2 lockout circuit 46 thus inhibits the firing of SCR24 between 0 and and between and 270 of the sine wave cycle. In otherwords, SCR firing line 58 is ungrounded only on the negative-going slopeof the rectified sine wave.

Voltage comparator 60 is comprised of transistors 62 and 64 having theiremitters connected together to a current source, as from a conventionalpower supply. The collector of transistor 64 is connected to ground, andits base is connected to signal conditioner 66. Signal conditioner 66has an input from line 20. As previously stated, line 20 has a rectifiedsine wave signal therein, which signal is in phase with a signalimpressed upon the input of SCR 24. Signal conditioner 66 has an outputto the base of transistor 64 which comprises a low amplitude invertedrectified sine wave displaced above zero voltage. For example, therectified sine wave has an amplitude of 1 volt peak to peak and it has apeak value of +7 volts with respect to ground. Thus, the base oftransistor 64 is connected to a reference voltage with positive spikes.When the base of transistor 62 is below the reference voltage,transistor 62 is conductive, giving a firing signal to trigger 56. Whenthe voltage on the base of transistor 62 is above the reference voltage,transistor 62 is cut off so there is no firing signal to SCR trigger S6.

Transistor 62 of voltage comparator 60 has its collector connected toSCR firing signal line 58, and has its base connected to theintermediate point of voltage divider comprised of resistors 68 and 70,Resistor 70 has its other end connected to ground In cases where thedesired voltage on energy storage capacitor 28, when the capacitor isfully charged, is fixed, resistor 68 can be of fixed value. However, inmost cases it is desirable that the voltage on the energy storagecapacitor 28, when it is charged to the desired value, be selectable todifferent values on different charges. In such a case, resistor 68 ismade variable as illustrated Variability is preferable.

The upper end of resistor 68 is connected to line 42 through diode 72and is connected to the positive side of energy storage capacitor 28through second signal line 74 and diode 76. The diodes 72 and 76connected face to face to the voltage divider comprised of resistors 68and 70 act as an analog signal comparator by comparing the voltages ofthese two signals and permitting the higher of the instantaneousvoltages to be impressed upon the voltage divider.

For pdrfidss'arim'snafih the operation JfThe capacitor charge circuit,and with respect to the table of components below, it will be assumedthat transformer secondary 14 is energized at 60 cycles per second andresults in a no load full wave rectified voltage in unregulated positivebus 20 of 400 volts, zero to peak, to charge capacitor 28 to a desiredmaximum of 300 volts. Furthermore, signal conditioner 66 has an outputof inverted full wave rectified voltage form with a 1 volt amplitude andis displaced at +7 volts peak. It is assumed that energy storagecapacitor 28 initially carries no charge.

The three examples given below illustrate, together with the .exemplaryvalues in the table below, the manner in which the charge circuit firesthe SCR at the optimum time to rapidly charge the capacitor 28, to bringthe capacitor up to the desired charge as quickly as is feasible. in thefirst example, the desired charge on the capacitor 28 is 7 volts. Thiscapacitor is initially uncharged, and thus line 74 has a zero charge init. Variable resistor 68 is reduced to zero ohms. It is understood thatwhen the base of transistor 62 is below the 7 volt reference, SCRtrigger 56 is fired Thus, with the valve of 700 ohms on resistor 70,when 10 milliamps or less is flowing in resistor 70, this condition ismet. Ten milliamps, in turn, on the 5 kilo-ohm resistor 44 means 50volts at the input of resistor.44, which is a point fairly well down thedownslope of the rectified wave in line 20. Thus, when the downslopevoltage goes down to 50 volts, the voltage on the base of transistor 62goes below 7 volts to cause SCR firing.

Because of copper losses and internal impedance in the transformer andthe line, the capacitor receives only a partial charge on the first halfcycle, even though the SCR was fired substantially 43 volts above thedesired charge on the capacitor. Charging continues on the followinghalf cycles until the voltage on line 74 rises to the 7 volts whichcauses cutoff.

In the second case, the desired charge on capacitor 28 is 100 volts.Now, variable resistor 68 is set to 9.3 kilo-ohms sosistor 70 is 700ohms, the divided voltage on the vase of transistor 62 is again 7 volts.With capacitor 28 uncharged, and with a peak voltage of 400 volts on theinput to resistor 44, current in excess of l0 milliamps flows throughresistor 44, resistor 68 and resistor 70 so that the current throughresistor 70 is in excess of 10 milliamps. In this condition, the voltageon the base of transistor 62 is in excess of 7 volts, and SCR trigger 56does not fire SCR 24. However, as the voltage applied to the input sideof resistor 44 goes down to 150 volts, with the decreasing voltage ofthe downslope of the rectified half wave, voltage division is such as tofire the SCR trigger 56. The total resistance of resistors 44, 68 and 70is 150 kiloohms, so that at l50 volts applied thereto, 10 milliamps isflowing. Since the voltage is goingdown, and 10 milliamps produces a 7volt equal to the reference, as the voltage goes down the downslope, thetrigger is fired. Thus, firing is accomplished at about 150 volts on thedownslope of the curve when a lOO-volt charge is desired. Again,successive charging on successive half cycles is accomplished until line74 reaches the desired lO0-volt value, which value produces the cutoffvalue of 7 volts on the base of transistor 62.

ln the third case, 200 volts is desired on capacitor 28. To obtain thisvalue, variable resistor 68 is set to 19.3 kilo-ohms so that the totalresistance through resistors 44, 68 and 70 is 25 kilo-ohms. Thus, at 250volts through this series connection 10 milliamps is passing throughresistor 70 which causes the application of 7 volts on the base oftransistor 62. Below that value SCR trigger 56 causes firing of SCR 24,and the voltage goes below that value on the downslope of the rectifiedsine wave. Accordingly, firing occurs on the downslope, when thereducing voltage on the rectified wave decreases below 250 volts, untilthe capacitor is charged sufficiently to raise the voltage of line 74 toa point where when compared to the voltage in line 42 as an analogsignal comparator, the voltage to the base of transistor 62 is above thereference point. Thus, firing continues in each half wave until feedbackfrom the charged capacitor signals sufficient charge.

There is a certain amount of leakage from energy storage capacitor 28through line 74, diode 76 and resistors 68 and 70. Recharging toovercome this leakage loss, and any other leakage loss, is accomplishedby the fact that the signal from signal conditioner 66 to the base oftransistor 64 is an inverted full wave rectified sine wave. This meansthat there are fairly sharp positive-going peaks. These peaks are phaserelated to the input of SCR 24 so that upon the occurrence of such apeak on the base of transistor 64, and consequent turnon of transistor62, SCR 24 is turned on only for a very short duration to pass enoughcurrent to only slightly raise the voltage on energy storage capacitor28, possibly only a few millivolts. This maintains the charge at theexact value.

After the desired charge is reached upon energy storage capacitor 28,the circuit is ready for a welding pulse. Electrodes 36 and 38 arepressed into engagement with workpieces 40 and weld control 78 isactuated. Weld control 78 is simply a circuit which prevents the firingof SCR 24 and causes the firing of SCR 32 through SCR trigger 80. Thus,weld control 78 can be a simple pushbutton switch which grounds out anyincoming firing signal to SCR trigger 56, and subsequently theretoconnects SCR trigger to a positive voltage firing source. It ISnecessary to prevent firing of SCR 24 during the welding operationbecause of the virtual short circuit on the output of SCR 24, when SCR32 is conductive. The turns ratio between primary 30 and secondary 34 issuch to cause adequate welding current between the electrodes andthrough the workpieces. After welding, SCR 32 is turned off and SCR 24is again permitted to charge energy storage capacitor 28.

Particular circuit components for the example given above are listed inthe table below. Of course, these components can be varied in accordancewith the varying needs.

TABLE 1 Ref. No. Component Identification l6 diode IN 4249 I8 diode IN4249 24 SCR 2N I850 28 capacitor l,000 uf 450 v 32 SCR cg: MCR 2935-730, 34 transformer turns ratio 500:l 44 resistor 5 K ohm 48 capacitor0.0l ufl KV 50 transistor 2N 2484 54 diode IN 3600 62 transistor 2N l132 64 transistor 2N l I32 68 variable resistor 0-30 K ohm 70 resistor700 ohm 72 diode lN 4249 76 diode IN 4249 This invention having beendescribed in its preferred embodiment, it is clear that it issusceptible to numerous modifications and embodiments within the abilityof those skilled in the art and without the exercise of the inventivefaculty. Accordingly, the scope of this invention is defined by thescope of the following claims.

What I claim is:

l. A capacitor charge and discharge circuit. said circuit comprising:

a capacitor, power supply means, a silicon-controlled rectifier, saidpower supply means, said silicon-controlled rectifier and said capacitorbeing serially connected. so that said silicon-controlled rectifiercontrols the charging current from said power supply means to saidcapacitor, said power supply means being adapted to supply electricvoltage between a maximum and zero;

'a discharge switch serially connected to a transformer primary tocomprise said discharge circuit, said discharge circuit being connectedin parallel to said capacitor, so that when said capacitor dischargesthrough said transformer primary, said discharge switch being openduring capacitor charging, the improvement comprising:

control means connnected to said silicon-controlled rectifier to controlthe amount of maximum charge upon said capacitor, said control meanscomprising:

a lockout circuit connected to said silicon-controlled rectifier topermit firing of said silicon-controlled rectifier into conduction onlyupon reducing voltage at the input of said silicon-controlled rectifier;

a voltage comparator having a reference voltage thereto;

a first signal means signaling the instantaneous value of voltage to theinput of said silicon-controlled rectifier, said first signal meansbeing connected to said voltage comparator so that when the voltage onthe input of said silicon-controlled rectifier decreases so that it isproportionately below the reference voltage on said voltage comparator,said silicon-controlled rectifier is triggered to charge said capacitor.

2. The capacitor charge circuit of claim I wherein said first signalmeans includes a pair of serially connected voltagedividing resistors,the point between said pair of resistors being connected to said voltagecomparator.

of resistors comprise one fixed resistor and one variable resistor sothat the amount of charge on said capacitor can be selected by variationof said variable resistor. 6. The capacitor charge circuit of claim 5wherein said volt age comparator has signal conditioner means connectedthereto, said signal conditioner means transmitting a spiked referencevoltage to said voltage comparator.

. 7. The capacitor charge circuit of claim (wherein said capacitor isthe weld energy capacitor in a welding circuit and said transformerprimary is a welding transformer primary connected across saidcapacitor, said transformer primary having an associated transformersecondary adapted to be connected to a workpiece so that discharge ofsaid capacitor through said welding transformer is adapted to cause alarge current discharge through a workpiece.

1. A capacitor charge and discharge circuit, said circuit comprising: acapacitor, power supply means, a silicon-controlled rectifier, saidpower supply means, said silicon-controlled rectifier and said capacitorbeing serially connected, so that said siliconcontrolled rectifiercontrols the charging current from said power supply means to saidcapacitor, said power supply means being adapted to supply electricvoltage between a maximum and zero; a discharge switch seriallyconnected to a transformer primary to comprise said discharge circuit,said discharge circuit being connected in parallel to said capacitor, sothat when said capacitor discharges through said transformer primary,said discharge switch being open during capacitor charging, theimprovement comprising: control means connnected to saidsilicon-controlled rectifier to control the amount of maximum chargeupon said capacitor, said control means comprising: a lockout circuitconnected to said silicon-controlled rectifier to permit firing of saidsilicon-controlled rectifier into conduction only upon reducing voltageat the input of said silicon-controlled rectifier; a voltage comparatorhaving a reference voltage thereto; a first signal means signaling theinstantaneous value of voltage to the input of said silicon-controlledrectifier, said first signal means being connected to said voltagecomparator so that when the voltage on the input of saidsiliconcontrolled rectifier decreases so that it is proportionatelybelow the reference voltage on said voltage comparator, saidsilicon-controlled Rectifier is triggered to charge said capacitor. 2.The capacitor charge circuit of claim 1 wherein said first signal meansincludes a pair of serially connected voltage-dividing resistors, thepoint between said pair of resistors being connected to said voltagecomparator.
 3. The capacitor charge circuit of claim 2 wherein a secondsignal means is connected between said capacitor and said voltagecomparator so that when said capacitor is charge to the desired level,said second signal means signals to said voltage comparator to preventthe first signal means from firing said silicon-controlled rectifier. 4.The capacitor charge circuit of claim 3 wherein said first and secondsignal means are connected to said voltage comparator through an analogsignal comparator.
 5. The capacitor charge circuit of claim 4 whereinsaid pair of resistors comprise one fixed resistor and one variableresistor so that the amount of charge on said capacitor can be selectedby variation of said variable resistor.
 6. The capacitor charge circuitof claim 5 wherein said voltage comparator has signal conditioner meansconnected thereto, said signal conditioner means transmitting a spikedreference voltage to said voltage comparator.
 7. The capacitor chargecircuit of claim 6 wherein said capacitor is the weld energy capacitorin a welding circuit and said transformer primary is a weldingtransformer primary connected across said capacitor, said transformerprimary having an associated transformer secondary adapted to beconnected to a workpiece so that discharge of said capacitor throughsaid welding transformer is adapted to cause a large current dischargethrough a workpiece.